Centrifugation of a biological or chemical sample in order to separate sample components requires high angular velocities. Generally, increases in angular velocity provide faster and/or more refined separations. A drive system of a centrifuge may be required to spin a sample-containing rotor at 100,000 revolutions per minute.
The drive system of the centrifuge is adapted for interchangeably mounting any of a variety of models of rotors onto a drive shaft. For a particular separation process, a rotor model is selected based upon the physical characteristics of the rotor model. The availability of a variety of types of rotors increases the versatility of the centrifuge in biological and chemical experimental research.
Each rotor model has a rated maximum safe speed, which generally depends upon maximum allowable centrifugally induced stresses. Operation in excess of the speed designed for safe operation of the rotor may lead to a catastrophic rotor failure. Such a failure may result in the rotor disconnecting from the drive shaft or in the rotor disintegrating into pieces. Additionally, a catastrophic rotor failure will typically render the entire centrifuge unusable.
There are a number of different known approaches to identifying rotors within a centrifuge. In a basic approach, the operator must input certain information before operation of the system is enabled. A concern with this approach is that the safeguard is subject to unintentional or intentional misidentification by the operator. Thus, industry regulations require further safeguards.
A second approach to rotor identification is operator independent. The rotor is caused to rotate within the centrifuge and spinning coding elements that are fixed to the rotor are optically read. The coding elements may be fixed to each rotor in a manner unique to the model to which the rotor is identified. A detection device within the centrifuge reads the coding elements and produces a rotor identification signal. Circuitry responsive to the signal ensures that the identified rotor is then maintained at or below the rated maximum safe speed. Coded rotors are described in U.S. Pat. Nos. 4,551,715 to Durbin and 5,221,250 to Cheng, both of which are assigned to the assignee of the present invention.
Indicative of a third approach to rotor identification is U.S. Pat. No. 4,827,197 to Giebeler, which is also assigned to the assignee of the present invention. Like the second approach, this approach is a back-up to the input of rotor ID by an operator. Giebeler teaches that a positive identification of a rotor may be made by calculating the moment of inertia of the rotor. The rotor is accelerated under constant torque. Acceleration from a first speed to a second speed is timed and the moment of inertia is computed by using the calculations of change in speed and change in time. After obtaining the moment of inertia, Giebeler teaches that the positive identification can be made by matching the calculated moment of inertia to a known moment of inertia of one of the rotor models.
U.S. Pat. No. 5,235,864 to Rosselli et al. also teaches using this third approach in which resistance to rotor acceleration is used to identify the rotor. However, instead of calculating moment of inertia, Rosselli et al. teaches using "windage," which is defined as the resistance to rotor motion that is a result of air friction along the surface of the rotor. Rosselli et al. teaches that a step in determining windage is either to measure the time needed to accelerate the rotor from a first relatively high speed to a second higher speed or to select a time period and measure the change in speed within the selected time period. The velocity signal or the time signal generated during this step is then used to generate a rotor identity signal by means of either comparing the signal with a reference signal indicative of a reference windage value or by means of addressing a look-up table of windage values. It is taught that in one embodiment a preliminary decision is made as to whether the rotor lies in the high windage regime or the low windage regime of rotors. However, it is left unclear as to how the decision is to be based. In any embodiment, the determination of windage is achieved by accelerating the rotor at relatively high speeds at which Rosselli et al. teaches that windage becomes dominant to inertia in resisting motion of the rotor.
A number of difficulties with identification schemes of the second approach, i.e., encoded rotors, are set forth in the Rosselli et al. patent. The coding elements and the decoder are located within the centrifuge and are subject to corrosion, which would adversely affect the ability of the system to accurately identify rotors. Moreover, it would not be possible to identify rotors that are not equipped with the coding elements. Retrofitting the coding elements onto pre-existing rotors or limited-use rotors would render the system susceptible to accidental or deliberate mismarkings.
U.S. Pat. No. 5,037,371 to Romanauskas describes an approach in which a transmitter emits a pulse of interrogating energy. The pulse is reflected by the rotor and is sensed by a receiver. The transmitter and receiver cooperate to generate a signature signal, or a signature signal pattern, based upon the distance traveled by the pulse of interrogating energy. The distance corresponds to the distance between the receiver and at least one, but preferably more than one, point on the surface of the rotor. Based upon the signature signal, an indicator signal is generated to represent the identity of the rotor. Using this approach, the rotor can be identified prior to rotation of the rotor. However, there are difficulties associated with this approach. Firstly, two rotor models may not be distinguishable if the rotors have basically the same dimensions. Secondly, because the transmitter and the receiver are located within the centrifuge, these elements are susceptible to sample spillage and other contaminants that enter the centrifuge housing. Moreover, the transmitter and receiver are fixed in place, so that designing rotors to predictably reflect the pulses of energy becomes an issue.
An object of the present invention is to provide a system and method for accurately identifying a stationary centrifuge rotor, wherein the equipment used for identification is protected from contaminants and the like.